Back to EveryPatent.com
| United States Patent |
6,168,637
|
|
Randolph
, et al.
|
January 2, 2001
|
Use of a large angle implant and current structure for eliminating a
critical mask in flash memory processing
Abstract
A method and system for providing a flash memory cell on a semiconductor is
disclosed. In one aspect, the method and system include providing a
plurality of gate stacks and providing a drain implant at an angle. The
plurality of gate stacks define a plurality of drain areas and a plurality
of source areas. The angle is measured from a direction perpendicular to
the surface of the semiconductor. The angle allows the plurality of gate
stacks to block the drain implant from reaching the plurality of source
areas. In another aspect, the method and system include providing a
plurality of gate stacks and providing a source implant at an angle. The
plurality of gate stacks define a plurality of drain areas and a plurality
of source areas. The angle is measured from a direction perpendicular to
the surface of the semiconductor. The angle allows the plurality of gate
stacks to block the source implant from reaching the plurality of drain
areas.
| Inventors:
|
Randolph; Mark (San Jose, CA);
Thurgate; Timothy J. (Sunnyvale, CA);
Luning; Scott D. (San Francisco, CA)
|
| Assignee:
|
Advanced Micro Devices, Inc. (Sunnyvale, CA)
|
| Appl. No.:
|
991322 |
| Filed:
|
December 16, 1997 |
| Current U.S. Class: |
29/25.01; 257/E21.682; 438/257; 438/302 |
| Intern'l Class: |
H01L 021/824.7 |
| Field of Search: |
438/257-267,302
29/25.01
|
References Cited
U.S. Patent Documents
| 5366915 | Nov., 1994 | Kodama.
| |
| 5674764 | Oct., 1997 | Liu et al.
| |
| 5770502 | Jun., 1998 | Lee | 438/264.
|
| 5891774 | Apr., 1999 | Ueda et al. | 438/264.
|
| 5920776 | Jul., 1999 | Fratin et al. | 438/257.
|
| 6008094 | Dec., 1999 | Krivokapic et al. | 438/286.
|
Primary Examiner: Booth; Richard
Attorney, Agent or Firm: Sawyer Law Group LLP
Claims
What is claimed is:
1. A system for providing a flash memory cell on a semiconductor
comprising:
means for providing a plurality of gate stacks, the plurality of gate
stacks defining a plurality of drain areas and a plurality of source
areas;
means for providing a plurality of drain implants, the plurality of drain
implants including at least one drain implant at an angle, the angle being
measured from a direction perpendicular to the surface of the
semiconductor, the angle being such that the plurality of gate stacks
blocks the at least one drain implant from reaching the plurality of
source areas; and
means for providing at least one source implant separately from each of the
plurality of drain implants, thereby allowing each of the at least one
source implant to be decoupled from each of the plurality of drain
implants.
2. The system of claim 1 wherein:
each of the plurality of gate stacks further includes a height;
each of the plurality of source areas further includes a length; and
the angle further has the property such that the tangent of the angle is at
least the length of each of the plurality of source areas divided by the
height of each of the plurality of gate stacks.
3. A system for providing a flash memory cell on a semiconductor
comprising:
means for providing a plurality of gate stacks, the plurality of gate
stacks defining a plurality of drain areas and a plurality of source
areas;
means for providing a plurality of source implants, the plurality of source
implants including at least one source implant at an angle, the angle
being measured from a direction perpendicular to the surface of the
semiconductor, the angle being such that the plurality of gate stacks
blocks the at least one source implant from reaching the plurality of
drain areas; and
means for providing at least one drain implant separately from each of the
at least one source implant, thereby allowing each of the plurality of
source implants to be decoupled from each of the at least one drain
implant.
4. The system of claim 3 wherein:
each of the plurality of gate stacks further includes a height;
each of the plurality of drain areas further includes a length; and
the angle further has the property such that the tangent of the angle is at
least the length of each of the plurality of drain areas divided by the
height of each of the plurality of gate stacks.
5. A method for providing a flash memory cell on a semiconductor comprising
the steps of:
(a) providing a plurality of gate stacks, the plurality of gate stacks
defining a plurality of drain areas and a plurality of source areas;
(b) providing a plurality of drain implants, the plurality of drain
implants including at least one drain at an angle, the angle being
measured from a direction perpendicular to the surface of the
semiconductor, the angle being such that the plurality of gate stacks
block the at least one drain implant from reaching the plurality of source
areas;
(c) providing at least one source implant separately from each of the
plurality of drain implants, thereby allowing each of the at least one
source implant to be decoupled from each of the plurality of drain
implants.
6. The method of claim 5 wherein:
each of the plurality of gate stacks further includes a height;
each of the plurality of source areas further includes a length; and
the angle further has the property such that the tangent of the angle is at
least the length of each of the plurality of source areas divided by the
height of each of the plurality of gate stacks.
7. A method for providing a flash memory cell on a semiconductor comprising
the steps of:
(a) providing a plurality of gate stacks, the plurality of gate stacks
defining a plurality of drain areas and a plurality of source areas; and
(b) providing plurality of source implants, the plurality of source implant
including at least one source implant at an angle, the angle being
measured from a direction perpendicular to the surface of the
semiconductor, the angle being such that the plurality of gate stacks
block the at least one source implant from reaching the plurality of drain
areas;
(c) providing at least one drain implant separately from each of the at
least one source implant, thereby allowing each of the plurality of source
implants to be decoupled from each of the at least one drain implant.
8. The method of claim 7 wherein:
each of the plurality of gate stacks further includes a height;
each of the plurality of drain areas further includes a length; and
the angle further has the property such that the tangent of the angle is at
least the length of each of the plurality of drain areas divided by the
height of each of the plurality of gate stacks.
Description
FIELD OF THE INVENTION
The present invention relates to flash memory and more particularly to a
method and system for decoupling source and drain implants without
requiring an additional mask.
BACKGROUND OF THE INVENTION
Conventional flash memory cells include a stacked gate, a source, and a
drain. Typically, the stacked gate includes a floating gate and a control
gate. In addition, the source and drain are typically not identical. The
source is conventionally used to erase the memory cell. For example, the
source is tailored to allow erasure through tunneling of charges between
the source and the gate. Thus, the source is formed such that a portion of
the source extends far enough under the gate stack to allow tunneling
during erasure. The source also typically has a higher concentration of
dopants than the drain. In addition, the sources of a large portion of the
memory cells in a sector of flash memory are electrically coupled. The
electrical coupling is typically provided by removing and isolation
structures separating the sources and implanting a dopant which
electrically couples the sources.
The drain is conventionally used to program the memory cell. Because
tunneling of charges is not used to program the memory cell, the drain
typically does not extend as far under the gate stack as the source. To
provide the program voltage electrical contact is made to the drain.
Typically, the area on which a drain is formed is longer than the area on
which a source is formed so that contact can be made to the drain.
In order to create a flash memory cell, the gate stack is typically formed
first. An implant for both the drain and source are then provided.
Typically, this implant is an arsenic implant. The drain and source
implant is coupled to reduce the number of masks required to form the
memory cell. The drain and source implant is typically provided at a
direction close to perpendicular to a surface of a semiconductor on which
the memory cell is being formed. For example, typical drain and source
implants occur at an angle of approximately seven degrees from
perpendicular to the surface of the semiconductor. Once the drain and
source implant is completed, a layer of photoresist is provided and
developed to mask the drain. Any further implants used to form the source
are then provided. Examples of such further implants include arsenic and
phosphorus.
Although the flash memory cell formed by the above process functions, it
would be beneficial to be able to further decouple the drain and source
implants. For example, if the drain and source implants were decoupled,
different energy implants could be used for the source and drain.
Different implant energies correspond to providing the implants at
different depths within the semiconductor. Moreover, if the drain and
source implant were decoupled, the drain and source could be better
tailored to their individual functions. However, introduction of an
additional mask should be avoided if possible to prevent complication of
processing.
Accordingly, what is needed is a system and method for decoupling the
source and drain implants without introduction of an additional mask. The
present invention addresses such a need.
SUMMARY OF THE INVENTION
The present invention provides a method and system for providing a flash
memory cell on a semiconductor. In one aspect, the method and system
comprise providing a plurality of gate stacks and providing a drain
implant at an angle. The plurality of gate stacks define a plurality of
drain areas and a plurality of source areas. The angle is measured from a
direction perpendicular to the surface of the semiconductor. The angle
allows the plurality of gate stacks to block the drain implant from
reaching the plurality of source areas. In another aspect, the method and
system comprise providing a plurality of gate stacks and providing a
source implant at an angle. The plurality of gate stacks define a
plurality of drain areas and a plurality of source areas. The angle is
measured from a direction perpendicular to the surface of the
semiconductor. The angle allows the plurality of gate stacks to block the
source implant from reaching the plurality of drain areas.
According to the system and method disclosed herein, the present invention
decouples the source and drain implants without requiring an additional
mask.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow chart depicting a conventional method for providing a
flash memory cell.
FIG. 2 is a is a flow chart depicting a method for providing a flash memory
cell in accordance with the present invention.
FIG. 3 is a block diagram of a flash memory during a drain implant in
accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an improvement in flash memory processing.
The following description is presented to enable one of ordinary skill in
the art to make and use the invention and is provided in the context of a
patent application and its requirements. Various modifications to the
preferred embodiment will be readily apparent to those skilled in the art
and the generic principles herein may be applied to other embodiments.
Thus, the present invention is not intended to be limited to the
embodiment shown but is to be accorded the widest scope consistent with
the principles and features described herein.
FIG. 1 is a flow chart of a conventional method 10 for providing a
conventional flash memory. A plurality of gate stacks are provided via
step 12. Typically, each gate stack includes at least a floating gate and
a control gate. The gate stacks define source and drain areas between the
gate stacks. Self-aligned source and drain implants are then provided via
step 14. In this step, a single implant is performed for both the source
and the drain simultaneously. This implant is typically an arsenic
implant. The source and drain areas between the gate stacks are,
therefore, doped. This self-aligned implanting step reduces the number of
masks required because both the source and drain are doped at the same
time.
If, as is typical, further source implants are used, then the drain of each
flash memory cell is masked via step 16. Typically, this masking is
accomplished by providing a layer of photoresist and developing the
photoresist layer to ensure that the drain is covered. Further source
implants are then provided via step 18. Further source implants may
include arsenic and phosphorus.
Although the method 10 shown in FIG. 1 provides a flash memory cell, those
with ordinary skill in the art will realize that the method 10 limits the
individualized tailoring of the source and drain. Typically, the source
and drain have different functions and, therefore, different structures.
For example, the source is conventionally used for erasing through
tunneling between the source and the floating gate. As a result, the
source extends farther under the floating gate than the drain. In
addition, the source is typically more heavily doped than the drain. The
source areas which are doped to become the sources are also typically not
as long as the drain areas because electrical contact is made to the
drain.
The present invention provides for a method and system for decoupling the
source and drain implants without requiring additional masks. The present
invention will be described in terms of providing a decoupled drain
implant and masking the drain to provide the source implant. However, one
of ordinary skill in the art will readily recognize that this method and
system will operate effectively to decouple a source implant without using
an additional masks. A method and system which decouples the source
implant without using additional masks would be particularly beneficial in
a system where the source was longer than the drain.
To more particularly illustrate the method and system in accordance with
the present invention, refer now to FIG. 2 depicting a flow chart of one
embodiment of such a method 100. Note that only relevant steps are
depicted in the method 100. Nothing prevents the insertion of additional
processing steps.
A plurality of gate stacks are first provided via step 102. In a preferred
embodiment, the each gate stack includes at least a floating gate and a
control gate. The plurality of gate stacks define a plurality of source
areas and a plurality of drain areas. A drain implant is then provided via
step 104. In one embodiment, this implant is an arsenic implant. The drain
implant is provided at an angle such that the source is not doped during
the drain implant. In other words, a dopant used in the drain implant
reaches the plurality of drain areas, but not the plurality of source
areas.
A mask for the source implant is then provided via step 106. In one
embodiment, the masking step 106 includes providing a layer of photoresist
and developing the layer of photoresist so that only the source areas are
exposed. The source implant(s) are then provided via step 108. These
implants could include arsenic and phosphorus implants.
FIG. 3 is a block diagram of a flash memory 200 during the drain implanting
step 104. The flash memory 200 includes a plurality of floating gates 202,
212, 222, and 232. Control gates 204, 214, 224, and 234 are disposed above
floating gates 202, 212, 222, and 232. Thus, the floating gates 202, 212,
222, and 232 and the control gates 204, 214, 224, and 234, respectively,
form gate stacks 201, 210, 220, and 230, respectively. The gate stacks
201, 210, 220, and 230 define source areas 240 and 244, and a drain area
242. Each gate stack 201, 210, 220, and 230 has a height h. Each source
area 240 and 244 has a length 1. Typically, the length of the source area,
1, is less than the length of the drain area, d.
By implanting the drain dopant at an appropriate angle 250, the drain 242
can be doped without doping the source areas 240 or 244. The angle 250 is
typically measured from a direction perpendicular to the surface of the
semiconductor. The drain 242 can be doped without requiring a mask on the
source areas 240 or 244 because the gate structures 201 and 220 shadow the
source areas 240 and 244, respectively, at the appropriate angle. This
angle is such that the tangent of the angle is at least the length of the
source area 240 or 244 divided by the height of the gate stack 201, 210,
220, or 230. Thus, the tangent of the angle is greater than or equal to
1/h. Although the implant is shown such that the gate stacks 201 and 220
shadow the source areas 240 and 244, respectively, nothing prevents the
use of the gate stacks 210 and 230 to shadow the source areas 240 and 244,
respectively.
As device sizes decrease, the lengths of the source areas 240 and 244 and
the length of the drain area 242 decreases. However, the height of the
gate stacks 201, 220, 230, and 240 does not significantly decrease. Thus,
the angle which allows doping of the drain area 242 while the source areas
240 and 244 are shadowed decreases as the device size decreases. As a
result, when the device size is small enough, conventional implanters can
be used to provide implant the drain area 242 while the source areas 240
and 244 are shadowed from the dopant.
In addition, the method 100 has been described for implanting the drain
area 242 partly because the drain 242 area is typically longer than the
source areas 240 and 244. Consequently, the angle which allows implanting
of the drain area 242 while the source areas 240 and 244 are shadowed is
smaller than an angle which would allow implanting of source areas 240 and
244 while the drain area 242 is shadowed. However, nothing prevents the
method and system from being used to implant source areas 240 and 244.
A method and system has been disclosed for decoupling drain and source
implants without requiring an additional mask. Although the present
invention has been described in accordance with the embodiments shown, one
of ordinary skill in the art will readily recognize that there could be
variations to the embodiments and those variations would be within the
spirit and scope of the present invention. Accordingly, many modifications
may be made by one of ordinary skill in the art without departing from the
spirit and scope of the appended claims.
Top